Issues of risk are characterized by uncertainty, and this uncertainty is inseparably technical and social or political: it results from the fact that “risks” are the object of attention of multiple actors, with diverging interests, assumptions and commitments. This is particularly true in the US, where technological hazards have been at the center of complicated regulatory configurations involving Congress, the White House, interest groups, NGOs, states… often split along political lines. Risk decision-making is a set of bureaucratic schemata that reduce this uncertainty. They organize evidence: its typified knowledge rubrics and processes produce knowledge that can be minimally accepted by the heterogeneous and conflicting set of actors involved in the regulation of the environmental and health hazards of technologies. They are adapted to, and evolve with these political configurations, as the various ways of handling uncertainties that have been devised at the US Environmental Protection Agency over time show. The paper offers a history of these changing modes of risk evidence, and shows how they shape, and are shaped by the politics of technology regulation.